Low molecular weight copolymers, and their salts, are useful as dispersants, scale inhibitors, detergent additives, sequestrants, etc. They can also act as crosslinkers for hydroxyl-containing substrates via esterification reactions or for amine-containing substrates via aminolysis reactions. Generally, molecular weights below 50,000 are necessary for effective performance, and often very low molecular weights, below 10,000, are most effective. It is common to use chain transfer agents in the polymerization reaction to produce low, and especially the very low, molecular weight polymers. Hypophosphorous acid or its salts (commonly sodium hypophosphite) are particularly desirable chain transfer agents, chosen primarily because they introduce phosphinate and phosphonate functionality into polymer molecules which confers superior performance properties in some applications. Phosphorous acid is also known for use as a chain transfer agent, although it is considerably less reactive than hypophosphorous acid. As used hereinafter, and in the appended claims, the term "hypophosphorous acid" is intended to include alkali metal, ammonium and amine salts thereof, unless a clearly different meaning is indicated.
The use of hypophosphorous acid and its benefits have been taught in several U.S. patents. U.S. Pat. No. 2,957,931 teaches the use of hypophosphorous acid to make organo-phosphorus compounds including organo-phosphorus telomers. U.S. Pat. Nos. 4,046,707, 4,105,551, 4,127,483, and 4,159,946 teach a method of inhibiting the formation of scale-forming salts using acrylic acid telomers made by hypophosphorous acid chain transfer. U.S. Pat. No. 4,621,127 teaches the use of hypophosphorous acid as a decolorizing agent in a polymerization process. Japanese Patent No. 02-34694 teaches the use of hypophosphorous acid chain terminated polymers and copolymers as detergent additives.
The use of hypophosphorous acid as a chain transfer agent results in the formation of several different polymer species. The reaction products include polymeric dialkyl phosphinate, polymeric monoalkyl phosphinate and polymeric monoalkyl phosphonate polymers. For certain applications, it is desirable to have either the di- or mono-alkyl derivative.
U.S. Pat. No. 4,681,686 ('686 patent) discloses cotelomers made using hypophosphorous acid as a chain transfer agent. The cotelomers are prepared from a wide range of comonomer types for use as corrosion and scale inhibitors, dispersants, and sealing smut inhibitors. The '686 patent discloses a preference for the polymeric dialkyl phosphinate products.
U.S. patent application Ser. No. 371,467 filed on Jun. 26, 1989, commonly assigned, teaches a method of increasing the efficiency of the incorporation of the hypophosphorous acid which subsequently results in an increased yield of the di-alkyl phosphinate species. In that application, the invention required from 20-100 percent in-process neutralization of the acid monomers in order to achieve efficient incorporation of the hypophosphorous acid.